The problems of foreign material deposition and corrosion and the attendant effects have troubled water systems for many years. For instance, deposits tend to form on the internal walls of various water systems such as boiler and cooling water systems and thereby materially lessen the operational efficiency of the systems.
Deposits may originate from several sources. For example, calcium carbonate, calcium sulfate, and calcium phosphate from a systems feedwater can accumulate along or around a systems metal surfaces forming scale. Also, manganese which can be found in natural waters at concentrations up to several parts per million can result in a formation of manganese deposits. Under chlorinated conditions, manganese (II) can be easily oxidized to the manganese plus 3 valence state which is extremely unstable. Brownish-black deposits of Mn.sub.2 O.sub.3 form under such conditions resulting in fouling of transfer lines and heat exchanger surfaces. The oxidation to insoluble manganese dioxide under chlorinated conditions is enhanced by elevated pH as can occur through cycling in cooling towers.
Manganese occurs at significant concentrations in many well and surface waters throughout the United States and is also present in estuarine and coastal waters. Manganese is sometimes referred to as "Ohio River Varnish" due to its ubiquitous presence along that river and the hard, dark brown to black nature of its deposits.
While manganese deposition has been noted on heat transfer and non-heat transfer surfaces, all metallurgies and plastics, metallurgy can play a factor in manganese deposition. Specific stainless steel alloys, admiralty and aluminum brass have all been found to be particularly susceptible to manganese deposition and the attendant pitting of the metal.
Any waters having detectable manganese concentrations (above about 0.02 ppm) may experience manganese deposition problems. At levels higher than 0.2 parts per million, deposition is very likely. Manganese levels greater than about 1 part per million generally cause several problems and manganese levels rarely exceed 2 parts per million even in cycled water.
Manganese deposition on tubes has been identified as manganese dioxide. Typically, it appears initially as a thin, tightly adherent, brownish-black varnish which becomes thicker, rougher and more irregular if permitted to accumulate. The uniform tightly adherent nature of the deposits lends itself well to setting up oxygen or metal ion concentration cells at holidays in the deposit probably responsible for the severe pitting effects of such deposits.
In once-through type cooling systems in Japan, injection of ferrous sulfate has been credited with greatly reducing malignant impingement attack of aluminum brass caused by manganese deposition. In the U.S., on-line mechanical cleaning devices as well as off-lie cleaning are often employed. Off-line cleaning procedures can include chemical acid cleaning, hydroblasting, and metal scrapper plugs.
U.S. Pat. No. 4,552,665, Ralston et al, discloses a method of stabilizing soluble manganese ions which comprises the addition of a copolymer of an unsaturated carboxylic compound and an unsaturated sulfonic compound. The copolymer, having a weight average molecular weight of less than about 50,000 and a weight ratio from 1:20 to 20:1 inhibits the precipitation of manganese ions and disperses the manganese reaction products in aqueous solutions.
In addition, to the copolymers disclosed in U.S. Pat. No. 4,552,665. Manganese ligands are often employed in aqueous systems such as cooling systems to help maintain manganese stabilization. Of the broad class of ligands which can be used for this purpose, polyacrylic acid is known to be an effective manganese stabilizer. Water soluble phosphates such as orthophosphates are also known to possess some manganese stabilization efficiency, however, the effect of orthophosphate is slight compared to the more effective ligands.
The stabilization of manganese under conditions where deposition is a problem can require relatively high levels of ligand treatment to adequately stabilize the manganeses ions.